The 3% Voltage Drop Rule, Explained
The 3% figure is a recommendation in an informational note, not a hard rule — but it is the number inspectors and equipment manufacturers expect you to design to.
Where the numbers come from
The recommendation is that branch circuits be designed so the voltage drop does not exceed 3%, and that the total from the service to the last outlet not exceed 5%.
Because it sits in an informational note, it is not enforced the way a mandatory rule is. In practice it still matters: motors overheat, electronics misbehave and equipment fails to start on long, undersized runs.
How to compute it
Voltage drop depends on current, one-way distance, conductor size and material. For a single-phase circuit the drop is twice the one-way run because the current travels out and back.
- Single-phase: VD = 2 x K x I x L / CM
- Three-phase: VD = 1.732 x K x I x L / CM
- K is about 12.9 for copper and 21.2 for aluminum
- CM is the conductor circular-mil area
What to do when you exceed 3%
You have three practical levers, in order of cost:
- Go up one or two conductor sizes (cheapest per unit, most common fix).
- Shorten the run (rarely possible after design).
- Reduce the load on that circuit.
Frequently asked
- Is 3% a code requirement?
- It appears as a recommendation in an informational note rather than a mandatory rule. Local amendments and project specifications frequently make it mandatory, so check the edition enforced in your area.
- Does voltage drop apply to feeders too?
- Yes. The 3% figure is usually applied to branch circuits, with 5% allowed for the combined feeder plus branch circuit.
- Why does the calculator ask for one-way length?
- The formula already doubles the distance for a single-phase circuit. Enter the distance from the source to the load, not the round trip.
Related calculators
Educational information only — not professional engineering advice. Reference table values are pending review; see the methodology page.